Dear ANDI: what should I wear to be cooler in sunny and hot weather? Testing radiative performance of apparel using an outdoor thermal manikin

Clothing properties, including color, fit, style, and textile composition, strongly influence human radiative heat gain, the dominant heat transfer pathway in hot, dry climates. Although textiles are increasingly engineered to reduce the heat load across the visible and near-infrared, and far-infrared regions, comparing realistic performance of such apparel is difficult because spectral measurements alone cannot capture interactions among the environment, clothing, and the geometry of the human body. Here, we evaluated apparel using an outdoor and internally cooled thermal manikin, “ANDI”, exposed to direct sunlight under hot, low-wind conditions. By dynamically matching the manikin surface and ambient air temperatures, convective heat transfer is minimized, enabling direct measurement of net radiative heat flux. We validated the approach by comparing the outdoor performance of white and black cotton and polyester shirts under natural or controlled airflow conditions, demonstrating high repeatability at air speeds near 1 m/s. Next, we evaluated how color, fit, style, and textile composition of upper-body garments, as well as headgear, influence radiative performance. Clothing color was the dominant factor controlling radiative heat gain, with white and light-colored garments substantially outperforming dark alternatives, whereas textile composition, garment fit, and style generally had smaller effects. Similarly, light-colored hats with thicker, opaque crowns provided the greatest reduction in head heat gain, whereas black hats consisting of thin fabrics offered considerably less protection despite similar coverage. These findings reinforce long-standing recommendations to wear light-colored clothing in the sun while also demonstrating the value of objective, whole-garment testing for comparing apparel designs and validating future radiative cooling technologies intended to improve outdoor thermal comfort and safety.